Heretofore, urethane polymers have been prepared by the addition of curing agents such as diols, triols, diamines, amino alcohols, and the like to urethane prepolymers. The addition was generally continuous, as over a short period of time, or in bulk, that is all of the curing agent added at one time. This would be true of "RIM" (reaction injection molding) processes in which polyols/diisocyanates/curing agent are continuously mixed and injected into a mold for curing as well as for "LIM" (liquid injection molding) processes in which a liquid urethane prepolymer is mixed with a curing agent and injected into a mold for curing.